1. Field of the Invention
This invention relates to an electromagnet for a charged-particle apparatus, and in particular, to the construction of a deflecting electromagnet.
2. Description of the Related Art
FIG. 1 is a plan view showing, by way of example, the charged-particle apparatus which was disclosed in "Superconducting Racetrack Electron Storage Ring and Coexistent Injector Microtron for Synchrotron Radiation" by Yoshikazu Miyahara, Koji Takata, and Tetsyta Nakanishi in the September 1984 issue of Technical Report No. 21 of the ISSP published by the Japan Chemical Engineering Information Center.
In the apparatus shown, charged particles are accumulated in an accumulation ring 1 constituting the charged-particle apparatus. These charged particles (e.g., electrons) are introduced into the accumulation ring 1 along an incident beam line 2. This apparatus is equipped with deflecting electromagnets 3 which are superconducting electgromagnets adapted to form an equilibrium orbit 4 by deflecting the charged particles and which are formed by combining deflecting coils as described below.
Radiation beam lines 5 are used for extracting radiations which are generated when the charged particles are deflected in the deflecting electromagnets 3. This radiation, which is called synchrotron radiation or SOR (synchrotron orbital radiation), is extracted and utilized for lithography, etc. Generally, a large number of radiation beam lines 5 are provided along the deflecting electromagnets 3 with a view to enhancing the efficiency of the apparatus. In the drawing, however, each deflecting electromagnet 3 is shown as provided with only one radiation beam line.
Four-pole electromagnets 6 are used to focus the charged particles in the accumulation ring 1, and six-pole electromagnets 7 are used to correct any non-linear magnetic fields or chromaticity of the deflecting electgromagnets 3. A high-frequency cavity 8 serves to compensate for the energy loss of the charged particles due to the emission of the ratiation, thereby accelerating them back to a predetermined energy level. A kicker magnet 9 shifts the equilibrium orbit 4 when introducing charged particles along the incident beam line 2, thereby aiding the introduction of new charged particles. A vacuum chamber 10 serves as a passage for the charged particles, an inflector 11 helps the charged particles to enter the accumulation ring 1 along the incident beam line 2, and a vacuum pump 12 serves to maintain a good vacuum in the vacuum chamber 10. These components are arranged along the equilibrium orbit 4. The vacuum chamber 10 has a high level of mechanical strength and is made of a stainless steel which may be readily baked to remove gases. An ultra-high vacuum is maintained on the inside of this vacuum chamber 10 by the vacuum pump 12, which prevents the charged particles from colliding with the gas molecules and losing energy, which would shorten their lives.
Next, FIGS. 2 to 4 are a perspective view, a plan view and a side view, respectively, showing one of the deflecting electromagnets 3 of FIG. 1.
The deflecting electromagnet 3 shown is composed of a pair of superconducting coils: an upper and a lower coil 31 and 32. Since these coils exert an ultra-high magnetic force, they adopt an air-iron core structure without iron cores. Arrows m.sub.1 and m.sub.2 indicate the direction of the electric currents in the coils 31 and 32, and arrow n indicates the direction of the electron beam on the equilibrium orbit 4. As is apparent from FIGS. 3 and 4, the equilibrium orbit 4 can be represented on a plane of a polar coordinate R.theta. (z=0) by a semicircle .rho..sub.0 and straight lines connected thereto. .rho..sub.1 and .rho..sub.2 indicate the inner and outer radii, respectively, of the banana-shaped coils 31 and 32.
Next, the operation of the conventional charged-particle apparatus shown in FIGS. 1 to 4 will be described.
The charged particles, introduced into the accumulation ring 1 along the incident beam line 2, are deflected in a pulse-like manner by the inflector 11, and their orbit is shifted by the kicker magnet 9. Thus, the charged particles circulate first along an orbit which deviates somewhat from the equilibrium orbit 4. After making several circuits, they come to circulate along the equilibrium orbit 4 in the direction indicated by arrow n. This equilibrium orbit 4 is determined by the manner of arrangement of the deflecting electromagnets 3 and of the four-pole electromagnets 6. The principal magnetic field generated in the upper and lower coils 31 and 32 by the electric currents in the direction m.sub.1 and m.sub.2 is in the -z (-y) direction, and the electric current flowing along the equilibrium orbit 4 is in the direction reverse to the electron-beam direction n. Accordingly, the charged particles, i.e., the electron beams, passing between the upper and lower coils 31 and 32 (in FIG. 2) receives an electromagnetic force in the -R direction in accordance with Fleming's left-hand rule and is bent with a curvature of the radius .rho..sub. 0. The radius .rho..sub.0 of this equilibrium orbit 4 can be expressed by the following equation: EQU .rho..sub.0 =P/(e.By) (1)
where P is the momentum of the electrons; e is the charge of the electrons; and By is the generated magnetic field in the y-axis direction of the upper and lower coils 31, 32.
The y-axis is an axis parallel to the z-axis and related to the equilibrium orbit 4, and the x-axis, which will be described below, is an axis in the same direction as the radius R of the polar coordinate with respect to the equilibrium orbit 4.
The high-frequency cavity 8 accelerates the charged particles, and the six-pole electromagnets 7 correct any unevenness in the radial direction of the magnetic fields of the deflecting electromagnets 3, any chromaticity, etc.
When the charged particles circulating along the equilibrium orbit 4 are thus deflected by the magnetic fields of the deflecting electromagnets 3, the electromagnetic wave due to the braking radiation is emitted as radiation from the radiant beam lines 5 in the tangential directions of the equilibrium orbit 4.
Since the electron beam is making a betatron oscillation around the equilibrium orbit 4, a uniform magnetic-field distribution (a good magnetic-field area) of about 10.sup.-4 to 10.sup.-3 is generally required in a direction perpendicular to the electron-beam direction n (mainly, the direction of R, i.e., the x-axis direction) over a range of several centimeters or more around the central orbit. In the case where the magnetic distribution of the superconducting deflecting coils 31 and 32 is uneven, the equilibrium orbit 4 of the electron beam deviates from the center of the upper and lower coils 31 and 32. If this deviation exceeds a predetermined value, the electron beam strikes the vacuum chamber 10 and is lost.
FIG. 5 is a characteristic diagram showing the distribution in the R (x-axis) direction of the magnetic field By in the deflecting electromagnet 3 as obtained by calculation. Supposing the inner radius .rho..sub.1 and the outer radius .rho..sub.2 of the upper and lower coils 31 and 32 to be 315.8 mm and 675.8 mm respectively, the diagram shows the value of (By-Byo)/Byo expressed as a percentage when the distance between the upper and lower coils 31 and 32 is 252 mm. Here, Byo represents the center of the equilibrium orbit 4, i.e., .omega.=50 mm. The radial position of the equilibrium orbit 4 of the R=.rho..sub.0 (x=0) obtained from the equation (1) is: EQU .rho..sub.0 =495.8 mm
As is apparent from FIG. 5, the position where the magnetic field By is at its peak is some position where the radius is somewhat larger than R=.rho..sub.0 (the outer side) when .theta.=90.degree.. The closer .theta. is to 0.degree., the nearer is the peak position to the side of the inner diameter .rho..sub.1 (the inner side). Thus, even if the equilibrium orbit 4 for the electron beam is fixed, the absolute value of the magnetic field to which the beam on the equilibrium orbit 4 is subjected varies considerably between the entrance of the deflecting electromagnets 3 and the central section. This variation is due to the banana-like configuration of the upper and lower coils 31 and 32.
FIG. 6 is a sectional view which shows, by way of example, a steering magnet in the charged-particle apparatus shown in "Designing UVSOR Storage Rings" No. UVSOR-9, December 1982, by the Molecular Science Institute.
In the steering magnet shown, an iron core 13 comprises a return yoke 14 and magnetic poles 15. A coil 16 is wound around the return yoke 14, and the above-mentioned magnetic poles 15 are arranged with a vacuum chamber 10 therebetween. Charged particles 17 pass through this vacuum chamber 10 along an equilibrium orbit 4.
FIG. 7 is a side view of the steering magnet shown in FIG. 6. The return yoke 14 has a width W.sub.1 of, for example, 100 mm, and the coil 16 has a width W.sub.2 of, for example, 300 mm.
Next, the operation of the steering magnet for a charged-particle apparatus having the above-described construction will be described. When electricity is supplied to the coil 16, a magnetic field is generated between the magnetic poles 15 in the horizontal or vertical direction, depending on the direction in which the magnetic poles 15 are installed. The steering magnet causes an electromagnetic force to be exerted in the direction of the vector product of the magnetic field generated between the magnetic poles 15 and the electric current due to the movement of the charged particles 17 passing between the magnetic poles 15, thereby slightly deflecting the orbit of the particles. Usually, steering magnets are used together with deflecting electromagnets 3 and four-pole electromagnets 6, etc. in a charged-particle accelerating ring, a charged-particle storage ring, etc. In such cases, all the steering magnets exhibit independent magnetic-field-output components, and the respective functions of these steering magnets with respect to the charged particles 17 are fixed independently.
The problem with the deflecting electromagnets in the conventional charged-particle apparatuses shown in FIGS. 1 to 5 is that the absolute value of the magnetic fields on the equilibrium orbit greatly varies from place to place, so that the equilibrium orbit for the electron beam suffers deviation. Furthermore, as shown in FIGS. 6 and 7, the electromagnets of conventional charged-particle apparatuses have the following problem: when, for instance, a single steering magnet is provided for each charged-particle storage ring, a space corresponding to the width W.sub.2 (about 300 mm) of the steering magnet has to be secured in the direction of the charged-particle orbit (see FIG. 7). Since several, in some cases ten or more, steering magnets are mounted on one storage ring or accelerating ring, the peripheral length of the ring has to be considerable, resulting in a very large ring.